The preparation of porous silica and zirconia particles with narrow particle size distributions, i.e., monodisperse particles, have received a great deal of attention, particularly since they have proven to be a useful packing materials for high performance liquid chromatography (HPLC). Furthermore, zirconia (ZrO.sub.2) has proven to be much more chemically stable at high pH (greater than about 13) than other inorganic HPLC column packings. See, for example, U.S. Pat. No. 5,015,373 (P. W. Carr et al.). For HPLC, the particles are preferably substantially spherical, generally uniform in size (preferably varying by only about 2 .mu.m), have a particle size of about 0.5-15 .mu.m (preferably about 3-10 .mu.m), and have pores ranging from about 100 .ANG. to 1000 .ANG. in diameter.
Various colloid-aggregation processes have been used for the preparation of porous zirconia spheres. One such method, an oil emulsion method, involves mechanically dispersing micron-scale droplets of an aqueous zirconia sol in an oil phase. Simultaneous gelation of the colloids within the droplet and extraction of water from the droplets yields zirconia aggregates that are further strengthened by sintering. This method has produced a polydisperse collection of spheres ranging from about 0.5 .mu.m to about 500 .mu.m in diameter. See, for example, U.S. Pat. No. 5,015,373 (P. W. Carr et al.); and U. Trudinger, Chromatogr., 535, 111 (1990). While surfactants can be used to help control the final particle size, zirconia spheres prepared by this method usually have a broad size distribution. Thus, size classification is mandatory to make these zirconia spheres useful for HPLC.
Techniques based on spray drying have also been used for the preparation of porous zirconia. See, for example, U.S. Pat. No. 5,015,373 (P. W. Carr et al.); and EP Pat. Appl. No. 0 490 226 A1 (1991). In this process a zirconia sol, which may contain a reactive binder, is forced through a nozzle. Droplets of zirconia solution are dried to yield rigid particles. This method has produced spheres ranging from about 1 .mu.m to about 100 .mu.m in diameter. Some size classification may be necessary to obtain particles of the desired diameter with a narrow size distribution, however.
Another method for the preparation of generally spherical, porous particles was first disclosed in U.S. Pat. No. 4,010,242 (Iler et al.). This method is referred to herein as "polymerization-induced colloid aggregation" (PICA) because polymer growth occurs along with colloid aggregation. It has also been referred to as coacervation or microencapsulation. In this method, urea and formaldehyde are mixed with an acidic colloidal sol. The urea and formaldehyde undergo acid-catalyzed polymerization, and the oligomer so formed adsorbs onto the surface of the colloids causing them to aggregate. See also, R. K. Iler, J. Colloid Interface Sci., 51, 388 (1975). Though the mechanism of aggregation is not clear, it may proceed by the formation of polymer linkages between colloids.
The potential advantages of PICA over other colloid-aggregation methods are: (1) it can yield particles with a narrow particle size distribution; and (2) it is relatively easy to implement. However, the method disclosed in U.S. Pat. No. 4,010,242 (Iler et al.) does not provide consistent results, particularly for the preparation of unclustered, generally spherical, porous ZrO.sub.2 particles. Thus, what is needed is a convenient synthesis method that will consistently yield appropriately porous and generally spherical particles, such as zirconia and silica particles, for example, with a narrow particle size distribution in relatively high yields. Also, what is needed is a synthesis method that allows greater control over the aggregation process and thereby tuning of the pore structure of the final particles.